Electrochemical fuel cells generate electrical energy by converting chemical energy derived from a fuel directly into electrical energy by the oxidation of the fuel in the cell. A typical fuel cell includes an anode, a cathode and an electrolyte. Fuel and an oxidant are supplied to the anode and cathode, respectively. At the anode, the fuel permeates the electrode material and reacts at the anode catalyst layer to form cations, which migrate through the electrolyte to the cathode. At the cathode, the oxidant (for example, oxygen or an oxygen containing gas supply) reacts at the cathode catalyst layer to form anions. The anions formed at the cathode react with the cations to form a reaction product. The fuel cell generates a useable electric current and the reaction product is removed from the cell.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or pure oxygen) as the oxidant, a catalyzed reaction at the anode produces hydrogen cations from the fuel supply. An ion exchange membrane facilitates the migration of hydrogen ions (protons) from the anode to the cathode. In addition to conducting hydrogen cations, the membrane isolates the hydrogen fuel stream from the oxidant stream comprising oxygen containing air. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to form liquid water as the reaction product.
The anode and cathode reactions in such fuel cells are shown in equations (1) and (2) below: EQU Anode reaction: H.sub.2 .fwdarw.2H.sup.+ +2e (1) EQU Cathode reactions: 1/2O.sub.2 +2H.sup.- +2e.fwdarw.H.sub.2 O(2)
Solid polymer fuel cells generally contain a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrodes formed of porous, electrically conductive sheet material. The electrodes are typically formed of carbon fiber paper, and are generally impregnated or coated with a hydrophobic polymer, such as polytetrafluoroethylene. The MEA contains a layer of catalyst at each membrane/electrode interface to induce the desired electrochemical reaction. A finely divided platinum catalyst is typically employed. The MEA is in turn disposed between two electrically conductive plates, each of which has at least one flow passage engraved or milled therein. These fluid flow field plates are typically formed of graphite. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes.
In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of water formed during operation of the cell.
Two or more fuel cells can be connected together in series or in parallel to increase the overall power output of the assembly. In such arrangements, the cells are typically connected in series. One side of a given plate serves as an anode plate for one cell and the other side of the plate is the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together by tie rods and end plates.
The stack typically includes feed manifolds or inlets for directing the fuel (substantially pure hydrogen, methanol reformate or natural gas reformate) and the oxidant (substantially pure oxygen or oxygen containing air) to the anode and cathode flow field channels. Exhaust manifolds or outlets are typically provided for expelling the unreacted fuel and oxidant gases, each carrying entrained water.
The stack also usually includes a feed manifold or inlet for directing the coolant fluid, typically water, to interior channels within the stack to absorb heat generated by the exothermic reaction of hydrogen and oxygen within the fuel cells. An outlet manifold allows the coolant water to leave the stack.
In fuel cells employing hydrogen as the active constituent of the fuel and oxygen as the active constituent of the oxidant, the fuel can be supplied as substantially pure hydrogen or as a hydrogen containing reformate as, for example, the product of the reformation of methanol and water or the reformation of natural gas. Similarly, the oxidant can be supplied as substantially pure oxygen or as oxygen-containing air.
The reactants are commonly humidified before they enter the stack so they will not dry out, and thus damage, the membranes separating the anode and cathode of each cell. Such membranes generally require the presence of water to effect ion transport.
The fuel cells are typically flooded with the selected fuel and the oxidant at a constant pressure. The pressure is generally controlled by a regulator at the source of the reactant. When an electrical load is placed on the circuit connecting the electrodes, the fuel and oxidant are consumed in direct proportion to the electrical current drawn by the load.
Each reactant stream exiting the stack generally contains the water added to humidify it. The oxidant stream exiting the stack also generally contains product water generated at the cathodes of the fuel cells. The excess water extracted from one or both of the reactant streams exiting the fuel cell is accumulated in a separator or knockout drum. The excess water can be recirculated and used as a coolant or drained from the system.
When one of the reactants used in the fuel cell is substantially pure hydrogen or oxygen, the unconsumed reactant exiting the fuel cell stack may be recirculated to minimize waste. After the excess water is removed from the unconsumed reactant, it is recirculated and merged with the fresh reactant stream upstream of the inlet to the fuel cell stack.
When one of the reactants is a dilute reactant, such as a reformate or air, the unconsumed portion of the reactant stream exiting the fuel cell stack may be recirculated, particularly if it is the fuel stream. However, the dilute reactant is more often discarded after it passes through the fuel cell once, particularly when the dilute reactant is air. The excess water in the unconsumed portion of the reactant is generally removed in a separator or knockout drum and then drained. The unconsumed portion of the reactant stream is then vented to the atmosphere.
It is advantageous to integrate the fuel cell product water stream with the coolant stream, and thereby use the product water generated electrochemically in the fuel cell stack to regulate the temperature of the stack. In this regard, the use of product water as the coolant avoids the need to provide a separate external source of coolant fluid, since the water generated by the cell is itself a suitable coolant fluid.
Now consider in particular a fuel cell system in which the hydrogen is recirculated in the system until it is substantially consumed entirely, while the oxygen is provided in dilute form as air. In one such system, the air is discharged after passing through the fuel cell one time, and before its oxygen content is substantially exhausted. In such a system it is useful to define reactant utilization ratios.
The oxygen utilization ratio is defined herein as the amount of the oxygen constituent delivered to the fuel cell per unit time divided by the amount of the oxygen constituent consumed in the fuel cell per unit time. More generally, a reactant utilization ratio can be defined. This ratio is defined herein as the amount of the active constituent of one reactant which is delivered to the fuel cell input per unit time divided by the amount of the active constituent of that reactant consumed in the fuel cell per unit time.
To avoid the inefficiency inherent in extracting the entire active constituent of a reactant supplied to the fuel cell, each reactant utilization ratio is generally maintained at a level substantially higher than 1.0. Exemplary oxygen utilization ratios for fuel cells are about 1.2 to about 3.0, preferably about 1.7 to about 2.2, and most preferably about 2.0. If the hydrogen or other fuel is recirculated and thus substantially completely consumed, the oxygen utilization ratio also represents the excess oxygen supplied, compared to the stoichiometric amount of oxygen consumed by the reaction with hydrogen to make water.
One way to improve the efficiency of a fuel cell power generation system is to optimize the reactant utilization ratios, and particularly the oxygen utilization ratio, in the fuel cell for the chosen operating conditions. (The hydrogen utilization ratio may also be optimized, within the scope of the present invention. In the illustrated embodiment as operated by the inventors, however, excess hydrogen is present and oxygen is the limiting reactant. Under these circumstances the inventors prefer to regulate the oxygen utilization ratio, rather than the hydrogen utilization ratio).
One consideration in optimizing the oxygen utilization ratio for a fuel cell is the power output of the fuel cell at any given time. In most practical systems, the fuel cell must have a variable power output so it can provide more or less power as needed. There is thus a need to optimize the oxygen utilization ratio, according to the transient power output of the fuel cell, to improve efficiency.
A complicating factor in optimizing the oxygen utilization ratio of a fuel cell is the variation in the amount of electrical power required to operate the fuel cell system under various electrical output and fuel cell operating conditions. Power is commonly diverted from the fuel cell's electrical output to operate the pumps, control systems, and other supporting apparatus of the fuel cell system itself. This diverted power is commonly referred to, and will be referred to herein, as "parasitic power. " The parasitic power requirement of the cell reduces the gross power output, as the parasitic power required to operate the fuel cell's supporting equipment must be subtracted from the gross power output of the cell to yield the net power available for delivery to the primary load powered by the fuel cell.
The amount of parasitic power required to operate the fuel cell system varies substantially with changes in the power output and other operating conditions of the fuel cell. For example, an increase in the net power output demanded of the fuel cell may also increase the amount of parasitic power which must be drawn from the cell to meet the increased net power demand. The gross power demand thus will increase more than the net power demand increase.
If the oxygen utilization ratio is high, as it typically is under high net power output conditions, the amount of parasitic power drawn from the fuel cell is high as well. This is particularly true for an ambient air-breathing fuel cell, where air is compressed before entering the fuel cell. Compression in an ambient-air system is usually accomplished by operating a compressor either fully or partially (i.e., supplemented by a flywheel or the like) on parasitic power. The amount of compressor parasitic power is proportional to the pressure and mass flow rate of the compressed air. If the oxygen utilization ratio is high, most of the air which is compressed, and particularly its inert nitrogen constituent, is not utilized in the fuel cell.
The parasitic power increase necessary to increase the gross power output of the fuel cell can be so great as to defeat the purpose of increasing the gross power output of the fuel cell. Much of the increase in the gross power output is lost to the increase in the parasitic power load under certain operating conditions.
On the other hand, when the net power demand decreases or is low, there is a corresponding need to reduce the parasitic power load required to operate the fuel cell. Otherwise, the fuel cell system will not be well adapted to conserve fuel when the net power demand decreases or is low.
Another problem in the art is how to provide a fuel cell power generation system having a substantially constant output voltage, even when its load current varies. Many electrical devices, particularly inverters for converting DC to AC power, require a substantially uniform voltage to operate efficiently, and to avoid damage if the variation in voltage is sufficiently large. However, as is demonstrated graphically in FIG. 1, in a fuel cell operated at a uniform pressure and temperature, the voltage output will change if the load (i.e., output) current changes. This presents a problem which must be solved if the fuel cell is to provide a constant voltage, notwithstanding changes in its output current.
Yet another problem in the art is how to independently and automatically regulate the pressure and mass flow rate of a reactant gas in a fuel cell power generation system. In prior systems, both the pressure and the mass flow rate of each reactant gas have been controlled upstream of the fuel cell, with the fuel cell and downstream apparatus representing a fixed resistance to flow (which, at the most, could be varied manually, as by adjusting an exhaust valve or the like). It has thus not been feasible to automatically vary the pressure of a reactant gas within the fuel cell, independent of the automatic regulation of the mass flow rate of the reactant gas within the fuel cell. For reasons which will become clearer later, it is often desirable to control these variables independently and automatically, so the fuel cell can respond optimally to variations in its electrical power output.
Accordingly, an object of the present invention is to optimize the reactant utilization in a fuel cell under various operating conditions.
Another object of the present invention is to provide a fuel cell power generation system having a substantially constant output voltage, even when its load current varies.
An additional object of the invention is to control the pressure of a reactant gas in the fuel cell in order to maintain a substantially constant voltage output.
Another object of the invention is to control the temperature in the fuel cell to maintain a substantially constant voltage output.
An additional object of the invention is to minimize the parasitic power drain in a fuel cell based electric power generation system, particularly when the system is operating at reduced net power demand levels.
Yet another object of the invention is to simultaneously regulate the pressure and mass flow rate of a reactant gas in an electric power generation system including at least one fuel cell.
One or more of the preceding objects, or one or more other objects which will become apparent upon consideration of the present specification, are satisfied by the invention described herein.